This invention relates to a tensioner used with a chain drive in an automotive timing application. More particularly, this invention relates to a tensioner that uses a dimpled or grooved section in the cover to provide lubrication without the loss of oil pressure.
Chain tensioning devices, such as hydraulic chain tensioners, are typically used as a control device for a chain drive system in an automobile timing system. Generally, it is important to impart and maintain a certain degree of tension to the chain to prevent noises or slippage. Prevention of slippage is especially important in the case of a chain driven camshaft in an internal combustion engine because slippage may alter the camshaft timing by several degrees, possibly causing damage. The tension in the chain may vary greatly due to the wide variation in the temperature and the linear thermal expansion among the various parts of the engine. Camshaft and crankshaft induced torsional vibrations cause chain tension to vary considerably. This tension variation results in chain elongation. Moreover, wear to the chain components during prolonged use can produce a decrease in the tension of the chain.
A hydraulic tensioner is used to take up the slack in the chain or belt that connects the camshafts to the crankshaft of the engine timing system or to tension the secondary chain that connects two camshafts in a dual overhead cam engine. A typical hydraulic tensioner is comprised of a housing having a bore, a fluid chamber defined by the bore, and a hollow piston biased in a protruding direction from the bore by a spring. A check valve is also included in the hydraulic tensioner to permit fluid flow from a source of pressurized fluid into the fluid chamber, while preventing back flow in the reverse direction. The force of the chain against the piston in an inward direction is balanced by the resistance force of the fluid and force of the spring in an outward direction.
Oil is supplied to the tensioner from a reservoir through the check valve. While the reservoir includes an opening to allow oil feed into the reservoir, other openings in the reservoir are typically closed by a plug that is press-fit into an opening to the exterior of the tensioner housing. The problems with the press-fitted plug in the reservoir are that the volume capacity of the reservoir is reduced and the reservoir is filled through an opening below the lower surface of the plug. Thus, flat plates or raised plates have been used as reservoir covers.
One example of a chain tensioner that utilizes such reservoir covers is described in Tsutsumi et al., U.S. Pat. No. 5,441,457, where a reservoir is formed as a hollow space in the housing and a flat or convex cover plate covers the reservoir. Oil is supplied through an oil supply bore. With a flat plate, the reservoir volume is equal to that of the hollow space within the housing. A convex cover plate, with its concave side facing the interior of the reservoir, provides a volume in the reservoir equal to the reservoir space formed within the housing plus the volume of the concave part of the plate. A small hole in the center of the plate permits air to escape from the top of the reservoir. If oil leaks out from the small hole, then it will flow back into the oil pan by gravity.
Similarly, in the present invention, the reservoir is formed within the tensioner housing and has a plate cover. However, the tensioner of the present invention has a depression in a flat cover that stiffens the flat plate. The depression does not contact the walls of the reservoir, but is provided in the center for strength purposes. An opening in the plate guides a jet or spray of oil from the reservoir onto the chain as it runs by the tensioner and also allows air to vent or escape from the reservoir.